I used to make wedding gowns for a living, and I've seen some showdowns in this regard. Also, my sister has been a wedding photographer for 35 years now, and in her files she has countless examples of bad bridal choices that would make your hair curl.
As a general rule, brides cannot be counted upon to be honest with themselves when it comes to how well a wedding gown flatters them. They almost always come in with a vision in their heads, and most of the time it is a vision in which they are 6 inches taller and 20 lbs. lighter than they really are. (Also, sometimes, 10 years younger, too.)
I always advised brides not to get a dress that they actively disliked, but to try to be open-minded about styles that they had never tried on, or styles that they might have only worn once when they were 15. Also, color is a HUGE issue. Most people cannot wear bright white very well at all, and I long lost count of the number of women who convinced themselves that all it would take was a tan. Nope -- it it makes you look sallow when you're pale, it will just make you look sallow AND grubby if you wear it with a tan. Another important issue with bridal gowns is that they will mostly be seen from a distance, so colored bands and trims can look very different in photos and from your guests' POV. I always remember that poor girl who got married in a televised ceremony in the MK a few years ago who had a brown sash on her gown, one that wasn't too much deeper than her skin tone -- in the photos it looked like the dress was two pieces with a bare midriff.
Up until the late 1970's bridal gowns were largely above fashion. There were "in" trends, but you could still choose from a really wide variety of styles and cuts to arrive at the very most flattering look for your body and coloring. No so now. Salons are completely given over to the trendy these days, and if you want something in a very different style you are going to have to go custom to get it.
Second opinions are important, and so is taking a photograph in the gown (easy to do now with cell phones). Sometimes a dress looks totally different in photos than it does in a mirror, and it is the photos that you will have to live with for the rest of your life. Still, too many second opinions are not good, either, because the party ends up taking sides; some will want to "stand up for" the bride even if she's making a poor choice, while others will not be able to get past their own taste to see the merits of a design that they themselves could never successfully wear.